Various types of car-mounted radar apparatuses utilizing, for example, an FM-CW method have been devised. For example, a radar apparatus according to Patent Document 1 includes one transmitting antenna and a plurality of receiving antennas evenly spaced apart. The radar apparatus receives a reflected wave by successively switching the receiving antennas during a modulation cycle of a transmission wave. An azimuth of an object is detected by calculating phase differences between reflected waves received each by one of the receiving antennas.
FIG. 9(A) is a block diagram of a radar apparatus in a related-art example 1 corresponding to Patent Document 1 in a case in which nine reception channels are realized. FIG. 9(B) is a diagram showing a reception-channel state with this structure. The radar apparatus in the related-art example 1 includes receiving antennas 101 through 109, a transmitting antenna 110, switch circuits 201 through 204, a VCO 301, a branch circuit 302, a LNA 303, a mixer 304, and an IF amplifier 305. In this case, the receiving antennas 101 through 109 are evenly spaced apart by a spacing d. The receiving antennas 101 through 109, which are evenly spaced apart by the spacing d, receive a reflected wave corresponding to a transmission wave transmitted from the transmitting antenna 110. As shown in FIG. 9(B), each of the reflected waves is assigned to a corresponding one of channels CH1 through CH9 that are arranged with a phase difference (2πdsin θ)/λ therebetween.
In addition, a radar apparatus corresponding to Patent Document 2 includes a plurality of transmitting antennas evenly spaced apart by a first spacing and a plurality of receiving antennas evenly spaced apart by a second spacing. A transmission wave is transmitted by successively switching the transmitting antennas, and the transmission wave is received by successively switching the receiving antennas for each of the transmitting antennas.
FIG. 10(A) is a block diagram of a radar apparatus in a related-art example 2 corresponding to Patent Document 2 in a case in which nine reception channels are realized. FIG. 10(B) is a diagram showing a reception-channel state with this structure. The radar apparatus in the related-art example 2 includes the receiving antennas 101 through 103, transmitting antennas 111 through 113, switch circuits 205 and 206, the VCO 301, the branch circuit 302, the LNA 303, the mixer 304, and the IF amplifier 305. In this case, the receiving antennas 101 through 103 are evenly spaced apart by the spacing d and the transmitting antennas 111 through 113 are evenly spaced apart by a spacing 3d. The receiving antennas 101 through 103 successively receive a reflected wave obtained from reflection of a transmission wave transmitted from the transmitting antennas 111, 112, and 113 that are successively switched. As shown in FIG. 10(B), each of reflected waves is assigned to a corresponding one of channels CH1 through CH9 that are arranged with a phase difference (2πdsin θ)/λ therebetween.
In addition, a radar apparatus according to Patent Document 3 includes a plurality of transmitting-and-receiving two-way antennas, and each of spacings between the transmitting-and-receiving two-way antennas is set to a predetermined value. A transmitting antenna is selected in synchronization with a modulation cycle of the transmission wave, and a receiving antenna is selected in a cycle shorter than the modulation cycle for the selected transmitting antenna.
FIG. 11 is a block diagram of a radar apparatus in a related-art example 3 corresponding to Patent Document 3 in a case in which eleven reception channels are realized. The radar apparatus in the related-art example 3 includes transmitting-and-receiving two-way antennas 401 through 404, switch circuits 501 and 502, the VCO 301, a coupler 302, the LNA 303, the mixer 304, and the IF amplifier 305. In this case, the transmitting-and-receiving two-way antennas 401 through 404 are arranged with spacings d, 2d, and 2d in this order therebetween. One of the transmitting-and-receiving two-way antennas is selected as a transmitting antenna and transmits a transmission wave, and the transmitting-and-receiving two-way antennas are successively selected as a receiving antenna and successively receive a reflected wave. In this way, as shown in FIG. 12, each of reflected waves is assigned to a corresponding one of reception channels. FIG. 4 in Patent Document 3 shows a diagram in which, as the number of channels, eleven channels have been allocated. This is because, in Patent Document 3, channel allocation of these channels is set using temporal differences. That is, since the channels are allocated using the temporal differences, the eleven channels having temporal differences therebetween are formed according to a signal, as a standard, received by the transmitting antenna. If this is set in terms of positional differences in space, the antenna 401 arranged at an end becomes a standard for a transmission and reception signal, and the number of channels and spacing between the channels as shown in FIG. 12 of the present invention are obtained.
Patent Document 1: Japanese Patent No. 3622565
Patent Document 2: Japanese Patent No. 3368874
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-3393
However, in the related-art example 1, the nine receiving antennas are necessary in a case in which nine reception channels are formed. In addition to these antennas, the transmitting antenna 110 is additionally necessary. Moreover, a plurality of stages of the switch circuits 201 through 204 used to select one receiving antenna from the receiving antennas 101 through 109 are necessary. Thus, such a radar apparatus is large, and loss of the reception signal increases.
In the related-art example 2, the number of antennas is decreased compared with the related-art example 1; however, spacings between the transmitting antennas 111 through 113 need to be set to be large, and such a radar apparatus is still large.
In the related-art example 3, with respect to a point that nine channels are realized, a small radar apparatus can be realized compared with the related-art examples 1 and 2. However, the channels are not set to be arranged with a uniform phase difference therebetween in the related-art example 3 since the transmitting-and-receiving two-way antennas are arranged with spacings whose ratio is 1:2:2. In the example shown in FIG. 12, channels CH8 and CH10 among the eleven channels are missing, and the phase difference between channels CH7 and CH9 is 4πd(sin θ)/λ and the phase difference between channels CH9 and CH11 is 4πd(sin θ)/λ. The phase difference between the other channels is 2πd(sin θ)/λ. Thus, nine channels that are not arranged with a uniform phase difference therebetween substantially need to be utilized. Therefore, the azimuth of an object is partially undetected, and azimuth-detection performance for object detection is low.